Gregory the Dialogist

The law cuts away sins of the flesh, but our Redeemer condemns even unlawful thoughts. If therefore they do not hear Moses and the prophets, neither will they believe if someone rises from the dead, because when will those who neglect to fulfill the lesser precepts of the law grow strong enough to obey the higher commands of our Savior? And indeed it is certain that those who refuse to fulfill his words undoubtedly refuse to believe him.

40 Homilies on the Gospels, Homily 40 ↗

Thomas Aquinas

After the prohibition of murder, adultery is forbidden. This is fitting, since husband and wife are as one body. "They shall be," says the Lord, "two in one flesh" [Gen 2:24]. Therefore, after an injury inflicted upon a man in his own person, none is so grave as that which is inflicted upon a person with whom one is joined. Adultery is forbidden both to the wife and the husband. …

Explanation of the Ten Commandments ↗

Bonaventure

And because the lower portion of the soul is informed by the higher and is given to it as a help, it is necessary that the lower have commandments by which it would be conformed to the higher, so that it behave piously, truly and holily. Piously toward superiors, most of all parents, hence, Honor your father and your mother. Truly and justly toward equals, hence: You shall not kill, which precludes any harm. Holily toward inferiors, hence, You shall not commit adultery, which forbids any act of impudicity.

Collations on the Hexaemeron, Collation 21 ↗